This invention relates to block copolymers of the ABA type wherein the A segments are poly(methyl methacrylate) and the B segments are polycarbonate. More particularly, the invention relates to such ABA copolymers having a number average molecular weight in the range from about 15,000 to 100,000 that are impact resistant, moldable, transparent and thermoplastic.
Poly(methyl methacrylate) (PMM) is used to mold automotive lenses because it is clear, abrasion resistant, has moderate tensile strength, is stable to UV radiation, and is easily injection moldable. However, poly(methyl methacrylate) has virtually no yield strength and is prone to impact breakage. It also has a relatively low distortion temperature that may be incompatible with the high output light bulbs used in compact lens designs.
Polycarbonate (PC) is also a clear thermoplastic used in lens molding. It has higher yield strength and impact resistance than PMM and a higher distortion temperature. However, PC is more expensive and difficult to mold.
It would be desirable to combine the best properties of PMM and PC in a single molding composition. I have found that merely blending poly(methyl methacrylate) and polycarbonate resins does not yield a mixture having the advantageous properties of either. Moreover, the blends are opaque rather than clear.
It is therefore an object of the invention to provide an impact resistant, injection moldable polymer containing both poly(methyl methacrylate) and polycarbonate. A more particular object is to provide a novel ABA type block copolymer wherein the A segments are poly(metyl methacrylate) and the B segment is polycarbonate. Another particular object is to provide such ABA type block copolymers wherein the molecular weight of the polymer segments is predetermined and in the range of 500 to 35,000 to yield a block copolymer having a number average molecular weight in the range of about 15,000 to 100,000.
Another object of the invention is to provide a method of making such block copolymers by reacting together linear polycarbonate B-segments having chemically functional groups at both ends with linear poly(methyl methacrylate) A-segments having a functional group at one end that is reactive with PC end groups to yield ABA type block copolymers. A more particular object is to react linear polycarbonate, terminated at both ends with --OH groups, with poly(methyl methacrylate), terminated at one end with a chloroformate group, to yield an ABA block copolymer with polycarbonate linkages between blocks. A more specific object of the invention is to provide a method of making such ABA poly(methyl methacrylate)-polycarbonate block copolymers where the molecular weights of the A and B segments can be controlled to yield block copolymers having desired physical properties.